1. Field of the Invention
The present invention relates to a color image processing apparatus and method, and a color image output apparatus, and more particularly to an apparatus and a method capable of presenting good color reproducibility.
2. Description of the Related Art (Relating to Color Space)
In output apparatuses such as color printers for printing figures and image data, color data to be handled are designated by RGB values, which are in turn designated by color modes or commands, for figure data, and are provided in the RGB point or frame sequential format for image data. Other than the RGB color space, YMC color spaces (such as depending on ink characteristics) specific to respective printers and XYZ color spaces or the like defined by CIE, etc. are also handled in some cases. In any case, actual printing is performed by making input data subjected in a printer to a color reproduction process (e.g., conversion from RGB to YMCK) corresponding to a color space defined for the printer when printed.
Generally, in consideration of color matching between color data handled by a color printer and a different apparatus (e.g., a color scanner or a color display such as CRT), it is often practiced to define one color space as a reference and perform color compensation suited to light emitting (coloring) characteristics for each color processing apparatus. In this case, color processing in the color printer is also executed to be compatible with the color space as a reference so that the color printer can output and reproduce an image displayed by a color display, for example, with good fidelity.
In a system shown in FIG. 1, if various apparatuses are each configured to be able to precisely perform color space conversion and color compensation, it is possible for those apparatuses to handle standardized colors. In an example of FIG. 1, for enabling a color scanner 11, a color display 13 and a color printer 15 to handle the same color data, one color space is defined as a reference and the color data are converted into color spaces specific to the apparatuses by associated color space converters 12, 14, 16. With such a configuration, the apparatuses can handle standardized colors.
In practice, however, the apparatuses have different ranges of color reproduction due to their intrinsic physical characteristics. Although it is difficult to pursuit for a calorimetric matching, color compensation for minimizing a color difference by the use of color difference formula represented by CIE 1976 L*a*b*, for example, is generally practiced in view of the above. Therefore, color printers are also designed to perform color space conversion and color compensation therein so as to print colors as possible as close to those on the color space as a reference when handling color data.
FIG. 2 is a block diagram showing typical data processing executed in a color printer, particularly showing a conversion process from input RGB data into YMC. As shown, data input through a receiving buffer 21 are supplied to a data analyzer 22 which analyzes a data type from the data format. In the case of image data, the data are branched to an image developing system 23, converted into YMC data by referring to a color reproduction processor 24, and then applied to a page buffer 26. In the case of computer graphics (CG) data, the data are branched to a CG developing system 25, converted into YMC data by referring to the color reproduction processor 24, and then applied to the page buffer 26 likewise.
While many color difference formulae have been proposed for methods of evaluating whether two colors represented on different media are identical to each other or not, any method has not yet been absolutely established and various methods are employed case by case depending on the purposes of use at the present. Similarly, there are several methods for color reproduction which are also employed case by case depending on the purposes of use. Taking into account color matching, the evaluating method is necessarily different depending on which color reproduction is aimed. In color printers, a color reproducing method executed therein is an important element directly affecting image quality of prints. Generally, as mentioned above, the color difference formula of CIE 1976 L*a *b* or the like is practiced so that color compensation is performed to minimize color differences. This method is effective when color data read by a scanner are reproduced by a color printer, because the original is a reflecting document (i.e., colors represented on paper) and reproducing those colors with ink used in the printer is relatively easy. In other words, as physical coloring mechanisms are basically the same, color reproduction is easier than the case of other media although there are problems of a difference in ink characteristics and ink density (gradation).
However, since colors produced by luminescence on screens of color displays are different in their physical characteristics per se from colors on a reflecting document, there are limitations in pursuing color reproduction by the use of general color difference formulae. When an image produced by such a display media is a natural picture, a method of color reproduction generally called preferred matching is often employed. This method is based on the concept of achieving good color reproduction for some important colors (e.g., color of the human skin) in an image, apart from whether a reproduced image and an original image have identical colors or not. Such color reproduction is effective in handling data such as a natural image, but a color reproduction process taking no consideration of identity of colors raises a drawback when handling data such as a CG image.
The above problem can be solved if a color reproduction process can be changed in accordance with data to be processed. In conventional color printers, however, the color processing referred in an internal developing process to produce a final output is fixed regardless of data to be handled, as shown in FIG. 2. Color printers are usually designed with a mind to achieve identity of colors. Accordingly, when data to be processed is a natural image, it was impossible to execute a different color reproduction process such as preferred matching.
The above-mentioned problems can also be thought below from another aspect.
Generally, in output apparatuses for forming an image based on color visual data input thereto, a difference in color reproduction range between the input side and the output apparatus is caused as a problem during a process of color reproduction. A CRT monitor or the like is a luminescent device in which light source colors RGB are mixed in an additive manner to carry out color reproduction. On the other hand, a printer or the like is a device in which ink colors YMCK are mixed in a subtractive manner to carry out color reproduction. Such a difference in the color reproducing manners results in different color reproduction ranges between the monitor and the printer and, usually, the monitor has a larger color reproduction range.
FIG. 8A shows how the color reproduction ranges are different from each other. Denoted by 80 in FIG. 8A is a color space which is theoretically conceivable in accordance with standards such as NTSC using an L*a* b* uniform color space, i.e., a color space which input data from a host computer can take. 81 is a color reproduction range of a monitor and 82 is a color reproduction range of a printer.
When an input color image is produced by an output apparatus, those colors which are outside a color reproduction range of the output apparatus are required to be made correspond to suitable colors in the color reproduction range. This is generally called color space compression. A conventional output apparatus incorporates only one kind of method for the color space compression or no methods, and the color space compression is performed on the host computer side.
There have generally been practiced several methods for the color space compression. These methods will now be described with reference to FIGS. 8A to 8D on an assumption that the color reproduction range of a monitor is converted into the color reproduction range of a printer.
In FIG. 8A, 83, 84 indicate the brightest colors (white points) in the respective color reproduction ranges of the monitor and the printer, whereas 85, 86 indicate the darkest colors (black points).
The first compressing method is performed by, as shown by a dot line in FIG. 8B, mapping the white and black points of the monitor to the white and black points of the printer, respectively, and then converting other colors while keeping the relative relationship to the white and black points. Thus, the conversion is made so that a figure indicated by the dot line is similar to the color reproduction range 81. This method is suitable for processing image data such as a photograph including many colors.
The second compressing method is performed by, as shown in FIG. 8C, mapping colors of the monitor to an outer edge of the color reproduction range of the printer without changing L* (luminosity). This method is suited to reproduce colors with high fidelity as desired for logo marks or the like (i.e., spot colors).
The third compressing method performs the compression by sacrificing luminosity to some extent without changing saturation as possible as practicable, as shown in FIG. 8C. This method is suitable for CG or the like.
Thus, since optimum methods for color space compression are different depending on the types of input data such as image data, spot colors and CG, it is desired to selectively employ any one of the compressing methods in conformity with the purpose.
As mentioned before, however, a conventional output apparatus incorporates, at best, only one kind of method for the color space compression, and cannot realize selective use of the above compressing methods depending on the types of input data. This leads to a difficulty in carrying out optimum color reproduction.
Further, an attempt of designing a host computer to select any one of the color space compressing methods and execute a compression process would result in a drawback of increasing a burden imposed on the host computer to execute the compression process and degrading performance of the entire system.